copy subset of Sprig template functions

docs/sprig/string_slice.md

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+# String List Functions
+
+These function operate on or generate slices of strings. In Go, a slice is a
+growable array. In Sprig, it's a special case of a `list`.
+
+## join
+
+Join a list of strings into a single string, with the given separator.
+
+```
+list "hello" "world" | join "_"
+```
+
+The above will produce `hello_world`
+
+`join` will try to convert non-strings to a string value:
+
+```
+list 1 2 3 | join "+"
+```
+
+The above will produce `1+2+3`
+
+## splitList and split
+
+Split a string into a list of strings:
+
+```
+splitList "$" "foo$bar$baz"
+```
+
+The above will return `[foo bar baz]`
+
+The older `split` function splits a string into a `dict`. It is designed to make
+it easy to use template dot notation for accessing members:
+
+```
+$a := split "$" "foo$bar$baz"
+```
+
+The above produces a map with index keys. `{_0: foo, _1: bar, _2: baz}`
+
+```
+$a._0
+```
+
+The above produces `foo`
+
+## splitn
+
+`splitn` function splits a string into a `dict` with `n` keys. It is designed to make
+it easy to use template dot notation for accessing members:
+
+```
+$a := splitn "$" 2 "foo$bar$baz"
+```
+
+The above produces a map with index keys. `{_0: foo, _1: bar$baz}`
+
+```
+$a._0
+```
+
+The above produces `foo`
+
+## sortAlpha
+
+The `sortAlpha` function sorts a list of strings into alphabetical (lexicographical)
+order.
+
+It does _not_ sort in place, but returns a sorted copy of the list, in keeping
+with the immutability of lists.